Polyamide 6 (PA6) is used in staple fibers and yarns in paper machine press felts due to its good mechanical properties. The crystalline modifications (alpha and gamma forms) and the degree of crystallinity of PA6 can change under stress (compression) and heat (friction), which are present during mechanical dewatering (nip). Here, we investigate the effects of the molecular weight and crystallinity of a polymer on the mechanical properties of PA6 staple fibers such as wear resistance and compaction. The experimental part involves the ageing of felt specimens in a wet pressing simulator. The properties of the felt (air permeability), and the durability and branching of the staple fiber are examined as functions of the wear of a press felt and the crystallinity of the polymer. The changes that take place in the PA6 polymer are investigated using several techniques, and their connection with the properties of a new or aged press felt is discussed. The results indicate that the increase in molecular weight has a beneficial effect on the material's mechanical properties, and our observations also support previous studies on the crystallinity and branching of PA6. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2222-2232, 2011